This invention relates to the detection and recording of biological pulses and blood flow, and particularly to ophthalmodynamometers.
For the last decade there has been growing interest in the use of ultrasound for medical diagnosis, especially in ophthalmology. A procedure has been developed for transmitting ultrasonic waves into the eye to obtain intraocular images as well as characteristic signals relating to intraocular structure and pathology such as tumors. One device for accomplishing these objects is shown in Carlin U.S. Pat. No. 3,371,660. A second procedure, known as Doppler Sonography, measures circulation in the ophthalmic vascular supply by measuring the Doppler frequency shift in an ultrasonic wave reflected from the circulating blood. Both ultrasonic procedures involve touching either the skin or corneal surface in order to minimize attenuation phenomena associated with ultrasonic propagation through air. Often a liquid medium is interposed between the ultrasonic transducer and the eye to match transducer-tissue impedances so that the maximum ultrasound is transmitted into the eye rather than reflected from its surface, one example being shown in Giglio U.S. Pat. No. 3,453,998.
A second area of interest is in the use of tonometry in diagnosing ophthalmic disorders, especially glaucoma. Tonometry covers a variety of procedures for measuring intraocular pressure and volume changes. A common method, electronic tonometry, involves placing a calibrated piezoelectric weight on the corneal surface to record intraocular pressure and volume changes to detect, for example, the elevated intraocular pressure caused by glaucoma. A local anesthetic is administered to a patient, an ophthalmologist trained in the use of an electronic tonometer is used, and the test takes about 5 minutes. Recently, tonometric procedures have been proposed to avoid touching the eye. In Lichtenstein et al. U.S. Pat. No. 3,545,260 a gas under pressure is utilized to depress the cornea. Ultrasonic pulses are then transmitted to and reflected from the cornea; the pulse-return time delay caused by the increased pulse path due to the corneal depression is then correlated with intraocular pressure to detect glaucoma. Another method, shown in Hobbs U.S. Pat. No. 3,613,666, involves placing an ultrasonic transducer next to the eyelid without touching the eye itself in order to vibrate the eye, transmitting a light beam to the eye, and detecting either a change in the amount of reflected light or a change in the direction of propagation of the reflected light, each caused by the eye vibrations. These changes are likewise correlated with intraocular pressure. In general, these tonometric techniques, as with the ultrasonic methods previously discussed, involve either directly touching the eye or applying to it some external force such as gas pressure or acoustical vibration.
A third area of medical interest is in ophthalmodynamometry, which involves measurement of the ocular pulse, as a diagnostic tool for a variety of ophthalmic and circulatory diseases. The physiology of the ocular pulse, a subject of much study, is explained as follows. The blood supply into the choroid layer of the eye is derived from the ophthalmic artery, which is itself a branch of the internal carotid artery. The internal and external carotid arteries arise from a bifurcation of the common carotid artery, the latter originating on the aortic arch. It is at or near the bifurcation of the common carotid artery that much vascular occlusive phenomena occur. As the heart beats, a pressure wave caused by the pulsating blood flow will arrive at the choroid layer, and generate a pressure pulse at the retina. This pressure pulse is propagated through the intraocular medium to the cornea. The cornea distends slightly (1-50 microns) in response, and reverts back to its resting shape in the diastolic portion of the heartbeat. This phenomenon is known as the ocular pulse. Although this motion of the eye is caused by the ocular pulse, the fine structure of this motion is a function of the elastic properties of corneal tissue, mechanical motions of intraocular medium, and spatial distribution of ocular blood flow.
Studies by Best et al. ("Graphic Analysis of the Ocular Pulse in Carotid Occlusion," Archives of Ophthalmology, March, 1971, vol. 85, pp. 315-319; "Ophthalmodynamometry and Ocular Pulse Studies in Carotid Occlusion," ibid., March, 1971, vol. 85, pp. 334-338; and "Ocular Pulse Studies in Carotid Stenosis," ibid., June, 1971, vol. 85, pp. 730-737) and by Horven and Nornes ("Crest Time Evaluation of Corneal Indentation Pulse," Archives of Ophthalmology, July, 1971, vol. 86, pp. 5-11) have shown that alterations in the ocular blood supply associated with carotid artery occlusions are reflected in distortions in the shape, amplitude, and duration of the ocular pulse. Conversely, it has been noted that relative differences between left and right ocular pulses are correlated with certain pathological states (e.g., glaucoma or abnormal cerebral circulation).
Specifically, it is desirable to obtain an accurate reading of the ocular pulse in its unbiased, natural state to get the maximum benefit from this physiological monitor. A sensitive pulse reading as an indicator of carotid blood flow can be used to detect, in its incipient stages, vascular occlusive matter, which, if allowed to continue, can cause emboli, leading to cerebral hypoxia, the phenomenon known as a "stroke." Ordinarily the detection of vascular occlusive matter has been accomplished by injection of a radio-opaque bolus into the carotid circulation followed by angiography. This procedure is not used until late in the occlusive process when candidates can be detected by symptoms.
Secondly, glaucoma can likewise be correlated to the ocular pulse before the patient becomes symptomatic.
Thirdly, elevated or otherwise abnormal circulation in the major arteries supplying the cerebral hemispheres will be reflected in the ophthalmic arteries and can be seen in changes in the parameters of the ocular pulse.
Finally, a wide range of other pathologies leading to distortions in the ocular pulse can be screened for by a careful, sensitive measurement of the pulse. Although discrimination between pathologies would require further diagnostic procedures as well as observation of clinical signs and symptoms, ocular pulse distortions would alert a physician to the necessity for additional tests. Exemplary of these other pathologies are orbital cellulitis, cavernous sinus thrombosis, retinopathies (arteriosclerotic and hypertensive), papilledema, and pulseless disease.
Measurement of the ocular pulse, as noted, is done by ophthalmodynamometry, which presently utilizes the tonometric techniques already discussed that involve touching the eye. See, e.g., Bron et al., "Tonographic Studies in Carotid Occlusive Disease," British Journal of Ophthalmology, 1967, vol. 51, pp. 577-595. Best et al. in the articles discussed above utilize a suction cup contacting the eye. Another method involves looking into the eye to watch the pulsation of the retinal artery. Visual methods also involve biasing the eye to produce a pulse large enough to be resolved visually. A third method that has been attempted to avoid contacting the eye is an interferometric technique in which motion due to the ocular pulse is ideally correlated with a changing light interference pattern.